I've been trying to implement the MS-20 filter from this paper http://lib.tkk.fi/Dipl/2010/urn100219.pdf
highlighted by Peter McCullough in a previous post about the MS-20 filter. Maybe someone has done it already I don't know. Aaaaanyhoooo my knowledge of these things is not great and I am stumped by the saturation part of the filter. Basically I've done something very wrong as the filter overloads pretty easy and resonance isn't in the correct place.